The demand for biosensors is increasingly growing these days. Usually, biosensors allow for the detection of a given specific molecule within an analyte, wherein the amount or concentration of said target molecule is typically small. For example, the amount of drugs or cardiac markers within saliva or blood may be measured. Drugs-of-abuse are generally small molecules that only possess one epitope and for this reason cannot be detected, e.g., by a sandwich assay. A competitive or inhibition assay is a preferred method to detect these molecules. A well-known competitive assay setup is to couple the target molecules of interest onto a surface, and link antibodies to a detection tag, that may be an enzyme, a fluorophore or magnetic beads. This system is used to perform a competitive assay between the target molecules from the sample and the target molecules on the surface, using the tagged antibodies. For road-side testing, the assay should be fast so that a test may be performed in about 1 min, and robust.
Generally, a biosensor device using frustrated total internal reflection (FTIR) comprises a sensor device in which a sensor cartridge is to be inserted. The sensor cartridge comprises a sensor chamber wherein at least a portion of a sensor surface or volume in said sensor chamber is prepared for the detection of the target molecules. Usually, the sensor surface includes various binding spots. The sensor cartridge may be a disposable polystyrene cartridge. Paramagnetic beads are arranged in the sensor chamber. To increase the reaction speed of target molecules in a liquid which is inserted into the sensor chamber, actuation means, such as actuation coils, are arranged below the cartridge to generate an actuation force to pull the beads towards the sensor surface. After a predetermined time, which should be sufficient for the beads to bond on the binding spots, the lower coil is switched off and thus the actuation force is removed. In order to pull the non-bonded beads away from the sensor surface, another magnetic field may be applied which is generated by another coil arranged above the cartridge. Subsequently, the presence of beads at the binding spots on the sensor surface may be detected. Usually, a predetermined coil current is applied to the coils in order to generate a predetermined magnetic field. The magnetic force applied by the coils may also be used to further manipulate the assay.
In a FTIR sensor device, a camera, preferably a CCD or a CMOS camera, may be used to image the light reflected from the sensor surface and to observe the binding on the binding spot on the sensor surface. A typical picture obtained with a FTIR biosensor device is shown in FIG. 1. In FIG. 1, the image of a sensor surface 11 is shown, the surface 11 comprising various binding spots A1, A2 which are surrounded by a white area B1 and B2. The pictures are obtained by substantially homogeneously illuminating the sensor surface 11 and projecting the reflected light via an optical system to the camera. The relative darkening of a binding spot, for example binding spot A1, compared to the surrounding white area B1 is a measure for the number of bindings. In FIG. 1, the situation is shown where the relative darkening of spot Al is greater than the relative darkening of spot A2. FIG. 1 further shows alignment marks 10 which define the positions of the binding spots.
Even thought the coil currents, and thus the generated magnetic field, may be controlled in a precise and reproducible way, the effect of the magnetic actuation applied on the magnetic beads depends on various parameters. For example, the assays may degrade over time which may change the composition of the matrix and the magnetic properties of the beads. The positioning of the cartridge in the reader and also the positioning of the actuation coils with respect to the cartridge and the binding spots may also change due to production tolerances when manufacturing the reader device and cartridge. The viscosity of the liquid applied to the sensor cartridge may vary since, e.g., different saliva samples which may be used, may have different viscosity. Moreover, the strength and quality of the chemical bindings may vary. For example, especially when measuring in blood, fragile bindings may occur, so that too low coil-currents will decrease the effect of the actuation, while too large currents may break the bindings or form clusters when non-bonded beads are to be pulled away from the sensor surface. The above-mentioned parameters may also vary depending on the temperature of the sensor device which may particularly change when the device is used for road-side testing. These parameters, which may strongly influence the magnetic actuation applied on the magnetic beads in the sensor cartridge, are difficult and expensive to control.